Microwave switching device



Oct. 13, 1959 R. F. SULLIVAN ET AL 2,908,878

MICROWAVE SWITCHING DEVICE 7 Filed May 27, 1955 OUTPUT INVENTOR Robe/f F. Sullivan Roy Conway LeCraw ATTORNEU United StatesPatent ce ,1 2,968,878 wncRowAvE swrrcmNG DEVICE Robert F. Sullivan and Roy Conway Le Craw, Washington, D.C., assignors to the United States of America as represented by the Secretary of the Army Application May 27, 1955, Serial No. 511,816 v S :1 Claim. (Cl. 333- 98 V tGranted under Title3 5, U.S. 'C0de 1952 sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This inventionrelates to devices for the substantially non-dissipative attenuation of microwave energy. In a typical application of the invention a ferrite .rod surrounded by a coil of wire is mounted in a cavity. At one end of. the cavity, which may be cylindrical, input means is provided for receiving microwave energy having a certain plane of polarization. At the other end of the cavity output means is provided, preferably responsive to energy having the same plane of polarization as the energy at the input end, for withdrawing energy from the other end. The geometry is preferably such that (1) thewave in the region of the ferrite rod is equivalent to the resultant of two circularly polarized components, (2) the cavity will propagate each circular component independently, (3) a bydetuning the cavity from resonance; and second, by

causing-rotationof the plane of polarization of the wave reaching the output end. High attenuations can be obtained as a result of these two eifects. The attenuation may be switched OE and on at high speeds.

An object of the invention is to provide a device for I controllably attenuating a microwave signal.

Another object is to provide a microwave attenuator having high attenuation.

Still anotherobject is to provide a fast-acting microwave switch having high attenuation.

1 Other objects, aspects, uses and advantages will betube 13, and coil 14 is supported axially in a cylindrical waveguide cavity structure 26 having an inner cylindrical cavity 27 by means of supports 21 which are preferably -of'foamed polystyrene or other low-loss material. Cavity structure 26 is provided with an axial input aperture 31 at input end 29, adapted to admit microwave energy from an external source into cavity 27. Another axial aperture 32 at the output end. 30 of cavity structure 26 is adapted to, permit microwave energy to flow out of caw'ty 27. Radial wire leads 41 and 42 are connected electrically to t-h'e respeetive'ends of coil'14 and yextend insulatedly 7 Patented Oct. 13, 1959 through apertures 43 and 44 in cavity structure 26. Choke structures 47 and 48 associated with apertures 43 and 44 are preferably provided, to inhibit the flow of microwave energy through apertures 43 and 44. Leads 41 and 42 are connected through a switch 52 to a battery or other power source 51.

With switch 52 openno current applied to coil 14 cavity 27 is resonant in the TE mode at a certain microwave frequency f Let microwave energy of frequency f and and having a certain plane of polarization be introduced through input aperture 31 into input end 29 of cavity 27. Let output aperture 32 at outputend 30 be coupled to a section of rectangular waveguide 61 or other structure adapted to accept microwave energy of frequency i having the same plane of polarization as the energy introduced into input aperture 31 and to discriminate against waves having other planes of polarization. It will be understood that under these conditions cavity 27 will function as a relatively effective coupling device and that energy will flow out of output aperture 32 at a I relatively high rate P It is a known characteristic of cylindrical waveguide excited in the T E mode that the wave along the axis of the guide may be broken down into two circular polarized components. It is also known that if a ferrite rod is placed axially in such a waveguide, and if an axial magnetic field is applied to the ferrite, the ferrite will have diiferent effective permeabilities for each circular polarized wave. (See C. L. Hogan, Bell System Tech. Jour., XXXI, No. 1 (1952).) It is known that as the wave progresses axially along the ferrite rod under such conditions the positive and negative circular polarized components are phase-shifted oppositely, and that the plane of polarization of the resultant wave becomes progressively rotated. Furthermore, the energy density of the TE mode in cylindrical waveguide is maximum along the axis and falls oif toward the walls; and in addition, because of the high dielectric constant of ferrites, the axial ferrite rod 12referring to the figure-further concentrates in the axial region the energy in cavity 27. The dielectric tube 13 also contributes to the concentration of energy.

Accordingly it will be understood that, if we now apply current to coil 14 by closing switch 52, an axial magnetic field will be applied to ferrite rod 12 and the plane of the TE wave passing axially through rod 12 will be rotated. Furthermore, because the input wave entering input aperture 31 effectively traverses cavity 27 longitudinally a number of times before leaving cavity 27 through output aperture 32, the rotation occurring in cavity 27 when switch 52. is closed is cumulative.

Moreover, it will be understood that, because the eifective permeability of rod 12 is changed for each circular component when switch 52 is closed, cavity 27 becomes detuned from the frequency f of the input wave.

Closing switch 52 thus has the dual effect of (1) reducing the amplitude and (2) rotating the plane of polarization, of the wave at output aperture 32. Accordingly, closing switch 52 reduces the energy flow from output aperture 32 into rectangular waveguide 61.

It will be understood that various mechanical and electronic equivalents for battery 51 and switch 52 may be utilized to apply current to coil 14, and that rapid switching cavity where-rotation of theplane of polarization of the TE modeoccurs, because radiationfrom the slot and phase shift due to radiation will vary as the plane of polarization changes.

Toikeepzeddy.-current IOSSGSL'dOWIi,C0111!-.ShQllldjllOlliHB too close: to\ the inner'wallof cavity structure:26; This'is one-of several considerations that determine? the. optimum diameters of rod.121and:tube13 in relation: toxthetinner diameter of structure .26.

Dielectric tube 13 may be omitted if desired; coil 14 may be wound directly on'ferrite rod 12. iHowever, we

prefer to include -dielectric tube 13 because we have found. that wev are thustableito obtain higher attenuations.

-We believe: that the; poorer performance when-tube .13 is omitted. is due. to mode 1ellipticity, and that tube :13 improves .theperformance 'by reducing the :energy. density in the vicinity.of:the.coil,.thuszreducing modeellipticity. -:It will-be apparent thatin certainjembodiments of our invention it maybe advantageous toprovide tunabilitytfor the cavity 27, andthat tunabilitycan readily -be provided stubs are in the same'plane and at right angles to each other, rojecting radially inward into cavity 27 from the walls of structure 26. Let stub'63a be parallel to the E vector with no currentapplied to coil 14. Suppose for the moment that stubt63b is omitted. With a particular adjustment of stub 63a, cavity 27 .is resonant at a certain frequency i when no current is applied to coil 14. App'lication of current to coil 14 will now cause rotation of theplane ofpolarization. Because the E vector is no longer parallel to stub 63a, stub 63a produces ellipticity of the rotated wave. This ellipticity is undesirable, tending to reduce attenuation. However, by adding stub 63b and adjusting its length we are able to minimize the ellipticity due to stub 63a and to maximize the attenuation.

Although we have described particular forms of our invention in which a ferrite body is located axially in a sisting of two such components. Also, materials other than ferrites may be found to have the property ofexperiencing the-necessary change of eifectivepermeability and causing the necessary rotation when subjected to an appropirate magnetic 'field.

It will be understood'that the amount of attenuation provided by the device in the figure will be a function of the magnetic'field appliedto ferrite rod 12 and thus of -the current applied to coil 14, and that this current and this field maybe readily varied to provide-various desired 'amounts of attenuation.

A particularly valuable application of the invention 'is as a high-speed on-off switch, and this application entails rapidly switching on and off the current applied to coil 14 and the resulting field applied to rod 12. Particularly for high-speed switch applications we prefer to keep the inductance of coil 14 low.

In one satisfactory switch model constructed in accordance with the invention we used a cavity structure 26 having an inner diameter of inch and a length of 3% inches. Apertures 31 and 32 were circular irises of 0.453 inch diameter. Ferrite rod 12 was a rod of a magnesium-manganese ferrite (Ferramic Rl) having a diameter of 0.23 inch and a length of 1 inches. Tube 13 was a Teflon tube of 0.625 inch outer diameter. Supports 21 were of a foamed polystyrene material having a dielectric constant closely approximating that vofair. Coil 14 consisted of 20 turns of No. 30 wire, single spaced;

.model was. found .to have a coil inductance .of .5 microhenries, an insertion loss of 0.8 db at the resonant frequency (approximately 9160 megacycles), a loaded Q of 200 (no current applied to coil), and a voltage standing wave ratio of 1.2. When coil 14 of this model was fed with 0.2 microsecond unidirectional pulses from a thyratron pulser the observedqswitching timev (switching time of the microwave energy from on to off .or from off to on?) was only 0.025 microsecond. Attenuation was 45 decibels above the insertion loss when the current was of optimum amplitude and was better than '31 decibels over a 2-to-1 current range.

Advantages of switches in accordance with our invention over known rotation-type ferrite switches include the following: (1) Our switches .providehigh aattenuation that is not critically dependent upon the amplitude of the magnetic field applied to the ferrite; known rotation-type ferrite switches are capable of.providing'isolation of the order of 30 db or more only whenthe magnetic field is held to'within a few percent of the optimum value. (2) Our-switchesxare less temperature sensitive; rotation .by a ferrite-is dependent on the stateof magnetization of the ferrite, which is temperature-dependent;the isolation provided by .our switchesis much less dependent on magnetic field, on state-of .rnagnetization, .and hence on temperature. (3) .Our switches can .be used .at .higher average power levels,.partly because'the .detuning of the cavity in our switches in the off condition means-that less power is .dissipatedin thetferrite, which is sornewhat lossy at microwave :frequencies.

.Among the useful;-applications that-we contemplatefor microwave switches -.m accordance :with our invention are the following:

-(1) .In two-antenna'transmitting receiving.systems, as a switch: to isolate the:receiver from the receiving antenna during period of transmission.

(2) ;Inone-antenna transmitting-receiving systems, as

.a replacement for the usual TR tube; one or more of our switches can be placed in the line inseries between the re- ;ceiver and the-antenna, and the switches can readily be switched off simultaneously with the application of transmitter power. Such an arrangement would have distinct advantages overtheusual T-R tube arrangement.

For one thing, our switches recover--i.e., switch from the off to the on'conditi onmuch faster than the usual TRtube, and the relatively slow recovery time of TR tubes is a serious limitation in some applications. Furthermoreflthe usual TR tubeunlike our switcheshas an uncertain shelf life becauseof susceptibility to envelope leaks andto envelope outgassing, so that TR tubesin systems-not used continuously must be checked periodically.

(3) In one-antenna systems using ferrite circulators, as a switch between 'the'circulator and the receiver, to

provide more isolation of the receiver from the transture being adapted to receive microwave energy having said predetermined frequency and a 1 predetermined plane of polarization, a microwave section coupled to said output aperture and adapted to accept microwave energy of saidpredeterminedfrequency having saidpredetermined plane of polarization .while discriminating againstwaves having. other planes of polarization, a ferrite rod-'coaxially mounted within said cavity, and electrical means for applying an axialmagneticafi eld :to said rod,

said electrical means comprising a coil within said cavity surrounding said rod, means for applying a current to said coil, and a tubular layer of solid low-loss material having a dielectric constant greater than that of air interposed between said ferrite rod and said coil, said cavity being adapted to resonate in theTE mode at said predetermined frequency in the absence of said magnetic field, and means for adjusting the resonant fiequency of said cavity comprising first and second adjustable tuning stubs projecting radially inward from the periphery of said structure, said stubs being coplanar and having their axes at right angles to each other, said-ferrite rod being so constructed and arranged in cooperation with said cavity that a large part of the energy that reaches said output aperture from said input aperture passes through said ferrite rod, said microwave energy applied to said input aperture thereby propagating into said microwave section at a relatively high rate in the absence of said magnetic field, the application of said magnetic field to said rod acting to simultaneously detune said microwave cavity and rotate the plane of polarization of said microwave energy passing through said rod so as to then cause relatively little energy to be propagated to said microwave section.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Van Trier: Experiments on Faraday Rotation of 20 Guided Waves, Applied Scientific Research, vol. 3, No. 2,

July 27, 1953, pages 142-144.

Fox et al.: Bell System Technical Journal, vol. 34, No. 1, January 1955, pages 3132. 

